1. Field of the Invention
This invention relates to internally nitrided ferritic stainless steel strip, sheet and fabricated products in cast or wrought form having good creep strength at elevated temperature, while retaining good room temperature formability, and to a process for production thereof. The strip, sheet and fabricated products further exhibit a low coefficient of thermal expansion, good sulfidation resistance and resistance to cyclic high temperature oxidation, these further properties not being possessed by the more expensive austenitic stainless steels. Although not so limited, the steels of the invention thus have utility in applications such as coal gasification, thermal reactors in automotive exhaust systems, gas turbine truck regenerators, early fuel evaporation valves, and the like. The material can thus replace the more expensive austenitic stainless steels in any application where its elevated temperature creep strength and oxidation resistance are equal or superior to those of austenitic stainless steels.
2. Description of the Prior Art
Case hardening by heat treating in an ammonia-containing atmosphere to form an iron-nitrogen structure which is transformed by quenching to produce high surface hardness has been practiced for many years. A typical process relating to nitriding of a "Nitralloy" type steel is disclosed in U.S. Pat. No. 3,399,085, issued Aug. 27, 1968 to H. E. Knechtel et al.
An article in "Heat Treatment of Metals" pages 39-49 (1975) entitled "Ferritic Nitrocarburising", reviews molten salt bath treatments of the cyanide and/or cyanate type, gaseous nitrocarburizing and vacuum nitrocarburizing. All such treatments, when applied to ferritic carbon steels and alloy steels, are indicated to result in formation of an epsilon iron carbonitride phase on the surface which improves tribological properties, fatigue resistance, and wear and anti-scuffing properties.
U.S. Pat. No. 3,847,682, issued Nov. 12, 1974 to Rollin E. Hook, discloses a method of increasing the yield strength of a low carbon steel by heating in an atmosphere comprising ammonia and hydrogen. A deoxidized, low carbon steel containing from about 0.002% to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, a nitride-forming element chosen from the group consisting of titanium, columbium, zirconium and mixtures thereof, in amounts such that titanium in solution is from about 0.02% to about 0.2%, columbium in solution is from about 0.025% to about 0.3%, and zirconium in solution is from about 0.025% to about 0.3%, an balance essentially iron, is heat treated at 1100.degree. to 1350.degree. F in an atmosphere containing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride. The preferred nitriding atmosphere comprises ammonia-hydrogen mixtures having 3% to 6% by volume ammonia. This patent further discloses that nitrogen taken into solid solution as a result of the alloy-nitrogen precipitation strengthening step can present weldability problems and can result in high ductile-to-brittle Charpy impact transistion temperatures. However, if the nitriding step is followed by a denitriding step, which involves annealing in hydrogen at about 1200.degree. F for at least two hours, the excess nitrogen is removed with a slight reduction in yield strength, thereby eliminating weld porosity and substantially reducing the ductile-to-brittle transistion temperature.
U.S. Pat. No. 3,804,678, issued Apr. 16, 1974 to L. E. Kindlimann, discloses an internally nitrided austenitic stainless steel preferably containing about 0.5% to 3% titanium which is caused to form dispersed nitride particles having an interparticle spacing of less than 10 microns, by nitriding within the range of 1600.degree. F to the melting point of the steel in an ammonia-or-nitrogen-containing atmosphere at super-atmospheric pressure. The balance of the atmosphere comprises a non-oxidizing or inert gas such as hydrogen or argon.
Other nitride formers disclosed in the Kindlimann patent are aluminum, vanadium, columbium, boron, zirconium, etc., but titanium is stated to be greatly preferred.
While the case hardening of ferritic stainless steel has been practiced in the prior art, and while the above-mentioned Hook and Kindlimann patents disclose the internal nitriding of low carbon steel and austenitic stainless steel, respectively, to the best of applicants' knowledge no previous attempts to subject ferritic stainless steels to internal nitriding have been successful in improving the elevated temperature properties thereof to a degree equal or superior to those of austenitic stainless steels. In view of the relatively high cost of austenitic stainless steels, and periodic scarcities of nickel, it is evident that there is a definite need for a relatively inexpensive non-nickel bearing material which exhibits good creep strength at elevated temperature and good elevated temperature oxidation resistance, together with good room temperature formability.